Field of the Invention
The present invention is related to electronics and, in particular, to AC/DC and AC/AC conversion.
Description of the Related Art
Energy Star mandates now require Power Factor Correction (PFC) for power supplies of 75 Watts and above. If trends continue, future requirements will be even more strict and will extend to lower-power devices.
Regulation of switched mode power converters is complicated by the phase shift implicit in an output filter. More filtration improves regulation under steady state conditions, but causes more delay in the feedback path. That delay complicates regulation under dynamic conditions. The performance of Pulse Width Modulated (PWM) converters is therefore a compromise between agility and stability. Morong et al. (U.S. Pat. Nos. 7,642,758 B2 and 7,965,064 B2) provide improved dynamic performance over PWM through prediction, but a real-time calculation burden is imposed.
One other technique to improve regulation is to load extra energy into the switched inductor. Bordillion (U.S. Pat. No. 6,552,917 B1) suggests energizing to an excess inductive current and recovering that excess energy on the primary side of the power converter at the end of each chopping cycle. That approach helps to solve the regulation problem, but incurs inefficiency because a portion of the inductive energy moves from primary to secondary, and then from secondary to primary to storage, without performing useful work.
Re-regulation is another approach. Placing a second, cascaded regulator after the first converter will surely improve regulation, but a second stage of power conversion may double the losses. A linear regulator can be employed, but reduced efficiency cannot be avoided. Others have proposed adding an auxiliary power supply to a flyback converter, including Webb et al. (U.S. Pat. No. 6,775,159 B2). The auxiliary supplies proposed are intended to power other circuitry, or to help produce the voltages needed for driving various power switches.
Most power converters with power factor correction (PFC) use a line filter followed by a diode bridge. These systems all incur diode losses in the bridge. A bridgeless, inductive, resonant approach is described by Cuk in U.S. Patent Application 2010/0259240 A1. Large inductors are needed to resonate at line frequencies. These converters have heretofore proven difficult in practice, so topologies using capacitive storage instead of inductive storage are seen as more desirable.
A preferred approach for higher power and higher efficiency PFC employs an active bridge, where two of the rectifier diodes are replaced by switches commutated at a frequency much higher than line frequency, allowing much smaller inductors. The active bridge has the advantage of removing one diode, and the associated diode drop, from the current path. Two diodes are eliminated if bipolar blocking switches are employed. For these reasons, active bridge systems have an advantage for performing PFC at high efficiency.
Conventionally, active bridge systems use a flyback stage regulated to perform PFC, producing an intermediate voltage of hundreds of volts stored in a capacitor. The stored energy is then down-converted using a buck converter to produce a regulated output. In such a system, all the power moves through the two cascaded conversion stages.
Several inventors have proposed a means of storing energy in a capacitor on the AC side of the isolation barrier. For example, in U.S. Pat. Nos. 6,952,354 and 7,061,776, Yang et al. propose adding an additional switch, an inductor, and three additional diodes to control the movement of power into and out of a storage capacitor in a single-stage topology. In addition to the extra complexity, all the power converted must traverse an extra semiconductor junction and the stored energy must pass through the extra inductor. In U.S. Patent Application No. 2004/0156217 A1, Phadke proposes adding an extra transformer winding, two diodes, and two extra switches in addition to the storage capacitor. Sufficient energy can then be stored in the capacitor to regulate the output voltage, but the storage voltage interacts with the AC line voltage to complicate the PFC control function. Also, since the flyback energy is divided between the output and the storage function, a mechanism must be provided to prevent the storage function from degrading the output regulation. Greater complexity or poor output regulation is the result. Others propose adding a second stage for re-regulation to address these shortcomings, but in so doing, defeat the purpose of building a regulated single-stage PFC controller.
There are examples in the prior art of single-inductor, multiple-output, switched-mode power converters. Li (U.S. Pat. No. 6,075,295), Caine (U.S. Pat. No. 4,847,742), and Gorder et al. (U.S. Pat. No. 5,617,015) describe controls to exactly balance the inductor energy loaded during the energize portion of the chopping cycle with the aggregate demand of the outputs. A limitation of non-predictive flyback or forward converters is that the energize termination is based on past or present conditions, but the energy transfer outcome depends on future conditions. That fact fundamentally limits regulation. The existence of multiple output voltages makes this form of regulation even more challenging. In addition, none of these multiple-output power converters have the capability to perform PFC.
Zero current switching is described by Vinciarelli in U.S. Pat. No. 4,415,959. Zero current switching is achieved by moving energy in discrete, quasi-resonant quanta.
AC powered converters increasingly require Power Factor Correction (PFC) for system efficiency and to conform to mandates. Power converters generally require an extra stage of power conversion to achieve near-unity Power Factor. There is a need for simple, efficient AC input power converters with near-ideal Power Factor that can replace physically larger and more costly two-stage converters.